Many modern printing devices incorporate thermal ink jet technology. Typically, this technology utilizes a printhead (also known as a pen) having a silicon die supporting one or more vaporization chambers. During operation, resistors or other ink ejection elements on the silicon die are heated in order to vaporize and eject ink through nozzles overlying the vaporization chambers, thereby causing dots of ink to be printed on a recording medium, such as paper.
In general, the temperature of a printhead will change or fluctuate while printing. When a printer is printing "light" areas or in a slow mode, the printhead temperature will drop; when a printer is printing "dense" areas or in a fast mode, the printhead temperature will rise. As the printhead temperature changes, it is important to ensure that the silicon die does not overheat. Overheating results primarily when the printhead has run out of ink (OOI) or because the printhead is experiencing a global de-prime (i.e., a disruption of the continuous flow of ink into a vaporization chamber due to a large air bubble caused by out-gassing or air-gulping). In such cases, the problem of overheating must be detected quickly and efficiently in order to avoid permanent damage to the printhead. Furthermore, if the printhead has run out of ink or is experiencing a global de-prime, printing must be interrupted to avoid the waste of paper.
Previous techniques set a fixed maximum operating temperature for the printhead; when the temperature of the printhead exceeded this fixed maximum operating temperature, a thermal shutdown was initiated for the printer. To avoid improper thermal shutdown when the printhead was simply getting warmer (for example, because the printer was printing a high density area or a in fast print mode), the fixed maximum operating temperature was required to be set conservatively high. Although such previous techniques were easy to implement, their effectiveness was limited only to cases where out-of-ink or global de-prime conditions occurred while printing dense areas of ink in fast mode. In fact, if either of the two failure conditions occurred while printing a low density area or while printing a high density area under a slow print mode, the printhead temperature would not increase sufficiently to trigger thermal shutdown. Accordingly, previous techniques did not adequately address the problems associated with overheating in a printhead.